A serious obstacle to the more ubiquitous use of protective eyewear (such as sunglasses and goggles) is that protective lenses can distort vision. This distortion has been thought to be caused by unwanted dioptric power or prismatic effects in the lens, which has been particularly severe in protective lenses that are designed to curve around the eye to the side of the head ("wrap") and/or tilt inward toward the cheekbone (pantoscopic tilt). Although wrap and tilt are aesthetically pleasing, and can provide superior physical protection of the eye, they can also cause the normal line of sight of the eye to strike the lens surface at an angle. This relationship has caused optical distortion that is distracting to the wearer, and presents a serious problem to persons who require precise visual input, such as athletes, pilots and surgeons. This distortion can also be problematic when performing even more common tasks.
The prior art is replete with examples of efforts to overcome optical distortion in protective eyewear. At first such lenses were made with concentric surfaces having no optical center or optical centerline, but the lenses had inherent minus power (which was considered undesirable), and excessive prismatic distortion along the line of sight and peripherally. Later lenses were made plano (zero power), and centered with the optic center at the geometric center of the lens aperture or eyewire, but the plano lenses were found to induce base out prism along the line of sight, and had poor peripheral optics.
Rayton's U.S. Pat. No. 1,741,536 (issued in 1929 to Bausch & Lomb) disclosed a protective goggle in which the front and back surfaces of the lenses were defined by two spheres of different radii having offset centers. An optical axis through the centers of the spheres was spaced from, and oriented parallel to, a line of sight. This optical configuration provided a tapered lens, in which the lens thickness gradually decreased from the optical center toward the edges. Maintaining the line of sight parallel to the optical axis helped neutralize the distortion that would otherwise be caused by wrapping the lenses laterally with respect to the eye.
In the 1980s, the Foster Grant Company sold dual lens Eyeguard protective eyewear, which held a tapering spherical lens in front of each eye with both wrap and pantoscopic tilt. The optical axis of each lens was horizontally and vertically spaced from, and maintained parallel to, the normal line of sight. This same concept was again claimed many years later in U.S. Pat. Nos. 5,648,832 and 5,689,323, which issued to Oakley, Inc. The parallel relationship between the optical axis and normal line of sight was found to be partially successful in minimizing optical distortion caused by wrap and pantoscopic tilt, but these lenses still had undesired peripheral performance, with prismatic effects that produced yoked and vergence demands.
U.S. Pat. Nos. 4,271,538 and 4,964,714 disclosed a similar position of the optical centerline in safety spectacles, where the optical centerline was horizontally and vertically displaced from, and parallel to, a normal line of sight. However, the '538 patent itself acknowledged that this relationship left a prismatic imbalance between the right and left eyes, that imposed a vergence demand on the eyes. Like the Foster Grant and Oakley eyewear, these lenses also suffered from undesired peripheral performance, with prismatic imbalance between the eyes that produced yoked and vergence demands.
In the correction of ordinary refractive errors such as myopia and hyperopia, the optical axis of a corrective lens may be slightly tilted from the normal line of sight. A slight downward decentration is commonly used in lenses that have pantoscopic tilt, to help keep the optical axis of the lens directed through the center of rotation of the eye. In a lens having 5-10 degrees of pantoscopic tilt, for example, the optical axis is often shifted about 3 mm below the normal line of sight. A deliberate decentration of a corrective lens may also be needed to compensate for misalignment of the eyes (such as phorias and tropias).
Decentered lenses may be manufactured by cutting a lens blank away from the geometric center of the lens blank. However, the periphery of an injection molded lens blank often includes optical irregularities, and those irregularities are incorporated into a lens which is cut from the edge of the blank. If the decentration is large, the dimensions of the lens must be small so that it can be cut from the lens blank. Alternatively, a larger lens blank can be used, but this solution leads to an inefficient use of large (and relatively more expensive) lens blanks. This problem is particularly acute for lenses manufactured in large quantities, where an incremental increase in the size of the lens blank can significantly increase the manufacturing cost.
It is an object of one embodiment of this invention to minimize optical distortion in protective and noncorrective lenses.
An object of an alternative embodiment of the invention is to provide a more efficient manufacturing method for decentered lenses.